Live › vulnerability › CVE-2025-43859
Go to dashboardThe h11 Python HTTP/1.1 library accepts malformed chunked-encoding bodies, skipping validation of the trailing CRLF after each chunk. When paired with a reverse proxy that parses chunked bodies differently, an attacker can smuggle a second request past the proxy and steal another user's session cookie or bypass access controls.
Summary written from the sources below. Check the originals before acting.
First seen by InfraStack 2026-10-27 16:35 UTC, 0h before NVD. All times on this page are UTC.
Applications running h11 at or below 0.14.0 (the advisory's affected range) are exposed, most commonly through servers or clients that depend on h11, such as uvicorn and httpcore. The risk needs a buggy proxy in front of the h11 service that disagrees on chunk boundaries, so a directly exposed h11 server with no such proxy is less at risk. Teams running only managed load balancers that normalize request bodies can rate the risk lower, but should still upgrade.
| Product | Ecosystem | Affected | Fixed in |
|---|---|---|---|
| h11 | pypi | < 0.16.0 | 0.16.0 |
pip show h11 or pip freeze | grep -i h11 and check whether the version is below 0.16.00.16.0 or later (the advisory says fixed in 0.15.0; the affected range in the source is below 0.16.0)Each row is the first time that source carried this signal. The analyst’s rows are its own findings.
A leniency in h11's parsing of line terminators in chunked-coding message bodies can lead to request smuggling vulnerabilities under certain conditions.
HTTP/1.1 Chunked-Encoding bodies are formatted as a sequence of "chunks", each of which consists of:
\r\nlength bytes of content\r\nIn versions of h11 up to 0.14.0, h11 instead parsed them as:
\r\nlength bytes of contenti.e. it did not validate that the trailing \r\n bytes were correct, and if you put 2 bytes of garbage there it would be accepted, instead of correctly rejecting the body as malformed.
By itself this is harmless. However, suppose you have a proxy or reverse-proxy that tries to analyze HTTP requests, and your proxy has a different bug in parsing Chunked-Encoding, acting as if the format is:
\r\nlength bytes of content\r\nFor example, pound had this bug -- it can happen if an implementer uses a generic "read until end of line" helper to consumes the trailing \r\n.
In this case, h11 and your proxy may both accept the same stream of bytes, but interpret them differently. For example, consider the following HTTP request(s) (assume all line breaks are \r\n):
GET /one HTTP/1.1
Host: localhost
Transfer-Encoding: chunked
5
AAAAAXX2
45
0
GET /two HTTP/1.1
Host: localhost
Transfer-Encoding: chunked
0
Here h11 will interpret it as two requests, one with body AAAAA45 and one with an empty body, while our hypothetical buggy proxy will interpret it as a single request, with body AAAAXX20\r\n\r\nGET /two .... And any time two HTTP processors both accept the same string of bytes but interpret them differently, you have the conditions for a "request smuggling" attack. For example, if /two is a dangerous endpoint and the job of the reverse proxy is to stop requests from getting there, then an attacker could use a bytestream like the above to circumvent this protection.
Even worse, if our buggy reverse proxy receives two requests from different users:
GET /one HTTP/1.1
Host: localhost
Transfer-Encoding: chunked
5
AAAAAXX999
0
GET /two HTTP/1.1
Host: localhost
Cookie: SESSION_KEY=abcdef...
...it will consider the first request to be complete and valid, and send both on to the h11-based web server over the same socket. The server will then see the two concatenated requests, and interpret them as one request to /one whose body includes /two's session key, potentially allowing one user to steal another's credentials.
Fixed in h11 0.15.0.
Since exploitation requires the combination of buggy h11 with a buggy (reverse) proxy, fixing either component is sufficient to mitigate this issue.
Reported by Jeppe Bonde Weikop on 2025-01-09.
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